Steel parts used in automobiles and various industrial machinery etc. are sometimes required to have fatigue strength at their surfaces. For example, in CVT pulleys for transmissions, wear resistance is demanded, while in gears, the fatigue characteristic of pitting resistance is demanded. For improvement of these characteristics, improvement of the surface hardness of the steel parts is considered effective. For steel materials, nitriding and soft nitriding are being increasingly applied. Nitriding and soft nitriding of steel materials are advantageous in that a high surface hardness is obtained and heat treatment strain is small.
Nitriding is a method of treatment that diffuses nitrogen into the surface of a steel material, while soft nitriding is treatment that diffuses nitrogen and carbon into the surface of the steel material. As the medium used for the nitriding and soft nitriding, there are gases, salt baths, plasma, etc. The transmission parts of automobiles are mainly treated by the excellent productivity gas nitriding and gas soft nitriding.
The hardened layer formed by the gas nitriding and gas soft nitriding is comprised of a nitrogen diffusion layer and a compound layer formed at the surface side from the nitrogen diffusion layer and of a thickness of several μm to several tens of μm. The nitrogen diffusion layer is a layer hardened by diffused nitrogen, solid-solution strengthening by carbon, and the particle dispersion strengthening mechanism of nitrides. It is known that improvement of the hardness and depth of the nitrogen diffusion layer gives rise to an improvement in the pitting resistance. In the past, therefore much research has been conducted into improvement of the hardness and depth of the diffusion layer. The compound layer is comprised of an ε phase mainly made of Fe2-3N and also containing carbon or a γ′ phase mainly made of Fe4N. Compared with a steel material, the hardness is extremely high. When the compound layer is formed, the wear resistance is improved.
As conventional findings relating to the compound layer and wear resistance, the following may be mentioned. PLT 1 proposes a gear part which has been nitrided or carbonitrided, has a content of nitrogen from at least the surface down to a depth of 150 μm of 0.2 to 0.8%, has a quenched hardened layer of a mixed structure of martensite and 10 to 40% of residual austenite, and has excellent pitting resistance and wear resistance. PLT 1 has a description relating to the nitrogen content at the steel surface, but has no description relating to the components, composition, and properties of a compound layer formed by nitriding.
Further, PLT 2 proposes a method of treatment using a mixed gas with a residual concentration of NH3 of 45 to 65 vol % for soft nitriding at a gas temperature of 530 to 565° C. for 2 hours to thereby form a compound layer of a thickness of 2 to 12 μm containing pores and improve the pitting resistance, wear resistance, etc. The compound layer described in PLT 2 is comprised of Fe3N (ε), Fe4N (γ′), etc.